The present invention relates to aircraft landing gears for large aircraft, and more particularly to an apparatus and method of attaching wheel and brake assemblies to multi-wheeled aircraft landing gears.
Description of the Prior Art Systems
In order to more fully understand the invention hereinafter described it is necessary to understand the present methods of transferring brake torque (from brake to stationary structure) for different types of landing gear e.g., such as in single and twin axle gears for purposes of illustration.
Single and Twin Axle Gears
The simplest method of reacting brake torque from the brake to a stationary part of the gear, is by means of shear bolts in a flanged mounted construction. Typical configurations are shown in FIGS. 1 and 2 for single and twin axles respectively. FIG. 3 shows a typical arrangement of these shear bolts 10 relative to the brake hydraulic actuators, 11 and is common to both FIGS. 1 and 2. A hollow axle 12 is used for both types of gears, and is prevented from rotating relative to the gear inner cylinder 15 by a lock pin 13.
Application of the present invention for these types of gears is impractical as there is no relative rotation between the pressure plate assemblies 28 and the gear inner cylinder 15, during gear retraction, and consequently brake compensating links are not used. In addition, there is probably no requirement for main gear steering for these types of gears.
Gears with Two (or more) Axles
The most common of these gears is the four-wheeled truck type, but the more recent six wheeled truck arrangements (shown in FIGS. 4 and 5) are types of gears more likely to be utilized as aircraft get larger and heavier.
Landing gears with 4-wheel trucks cannot have rigid flange mounted brake connections, due to the rotation of the truck assembly 16 relative to the inner cylinder 15 during landing, taxiing, and during retraction. (Differences between FIGS. 4 and 6 illustrate this rotation.) This also applies to the fore and aft axles of 6-wheeled trucks. In these cases, the brake torque for each individual brake, is transmitted to the non rotating inner cylinder 15 by means of a pin jointed link, generally known as a brake compensating link 17 Fore and 17 Aft.
The brake compensating link pin joints are shown as 20, 21, and 22 in FIGS. 4 and 5, and are of course, left and right handed. In most brake designs the brake xe2x80x9cStatorxe2x80x9d assemblies 27 and 29 includes the brake pressure plates 28 which contains the brake hydraulic actuators, 11 and is held stationary against rotation (around the axle) by the Compensating Links 17 fore and 17 aft, during the braking operations. The pressure plate assembly, (although located on the axle, is allowed to revolve on that axle as the angle xe2x80x9cxcex8xe2x80x9d varies during the gear retraction (see FIG. 6).
Problems with Prior Art Systems When Main Gear Steering is a Requirement
In order to meet the main gear steering requirements, brake compensating links 17 fore and aft, have to align with the steered wheels (see FIG. 8). Such a steering angle (20xc2x0 minimum) would be in excess of the angular movement of ball joints are used in brake compensating links, and which usually have operating limits of +/xe2x88x9215xc2x0 Max.
The presence of a conventionally installed brake compensating link 17 restricts the inboard excursion of the tire during main gear steering (see FIG. 8).
Full efficiency of brake compensation is not maintained when braking and steering occur simultaneously. Brake compensating links axle geometry deviates from a true parallelogram as the steering angle increases.
U.S. Pat. No. 3,403,875 (Hartman) discloses a landing gear in which the brake is mounted on the end of a non-rotating axle stub where the wheel assembly slips over the brake and axle stub, engaging the rotating brake disks by splines on the inside of the hollow axle. The wheel bearing is mounted around the axle stub and the wheel bolts to the outer race of the bearing.
U.S. Pat. No. 4,659,040 (Sinclair) discloses a landing gear truck in which the two rear wheels can swing relative to the front wheels to allow steering at relatively small radii without excessive tire scuff. In this braking system one wheel is fixed to a rotatable common axle while the other wheel is free to rotate about the axle. The braking system is all concentrated in the vicinity of the free wheel where the free wheel is braked and the axle is braked thus braking the other wheel.
It is an object of the present invention to provide a structure between adjacent wheel brakes which rigidly joins the right-band brake stator 27 to the left-hand brake stator 29 (see FIG. 9). This structure is then capable (when assembled to the gear truck beam 16) of rotating in the plane of axle rotation during retraction, and reacts the brake torque by means of a single compensating link 31, per pair of wheels. A single compensating link 31 permits larger steering angles compared to a conventional double link arrangement, due to the flexibility of its installation position, and its independence of the steerable components. If for installational reasons, a double link is necessary, the present invention would still favor larger steering angles.
In contrast to prior systems having a maximum steering angle of xc2x18xc2x0, the present axle/brake plate integration removes at least this constraint. The present system""s ability to move links toward the center of the truck, results in increased clearance between the tire and the compensating rod consequently allowing more steering capacity, about an additional 7xc2x0.
More importantly, the single compensating links"" geometry, being independent of the wheel steering angle, maintains the characteristics of a parallelogram with the wheel axles, even during steering. Present systems cannot achieve, this completely as the conventional compensating link tries to lengthen or shorten, depending upon which direction the wheel is being steered, due to one end of each link being fixed. (See FIGS. 8 and 8A particularly with regard to components 22/22A, and 20/20A.)
Although the effect is undoubtedly small, the inability of the conventional geometry to maintain a parallelogram with the axles, induces out of balance forces and moments to the truck beam and links during steering. The present invention eliminates this possibility.
The word xe2x80x9cparallelogramxe2x80x9d is partially defined in FIGS. 4 and 6. The parallelogram is described in those two figures by the points 20, 23 and 21, 24, and 22, 25. The lengths between brake rod points (20 and 21), and (21 and 22), are identical to lengths between axle points (23 and 24) and (24 and 25) respectively, and the distances between (23 and 20) and (24 and 21) and (25 and 22) are all identical also, and is therefore a parallelogram.
This configuration remains a parallelogram no matter what attitude the main cylinder (15 and 30) relative to the centerline connecting the axles (25, 24 and 23) happened to be.
It is desirable that point 24 (the point of rotation between the main cylinder (15 and 30) be on the same waterline as that of the axles 25, 24 and 23. If, for other reasons of design, point 24 is not on the desired waterline, then an out of balance turning moment occurs in the truck when the brakes are applied, and the result is such that there is an ever increasing tendency particularly in the taxiing mode, for the front axle to become overloaded, and the rear axle to lift off the ground. This situation can be overcome by positioning the brake rods such that the instantaneous centers of both the rods and axles intersect each other at the static ground line.
Unfortunately, this process allows a truck to be fully balanced only when the gear system is in the static position which is the most important case. However, for all other gear attitudes (usually during gear retraction or extension), dampers can be used to reduce or eliminate any unbalanced moments on the truck when the brakes are applied. Such dampers are used extensively, but usually for truck positioning purposes only. Their function as a means of reducing, or eliminating this unbalance moment is probably not taken into account.
The reason that the single brake rod would be preferred is that its positioning (nearer to the C/L of the truck) allows the wheel and tire assembly more angular movement, (i.e., xc2x115 degrees approx. max.) This angle is sometimes less, depending upon the wheel well door opening size or the strength of character of the gear designer.
It must be understood that tire, wheel and brake rotors rotate from the start of an aircraft""s takeoff roll, to the time when either internal friction overcomes the momentum of the mass of that assembly when airborne, or the pilot applies the brakes prior to the gear entering the wheel well. This means that for the latter, (and for all instances of brake application) the torque that develops at each brake must be reacted by two equal but opposing forces, acting parallel to each other. Both of these forces leave or enter the first available stationary structure (main inner and outer cylinders), one via the compensating link (single or double), and the other via the truck itself (see FIG. 4A).